Bellows pipes, or flexible tubing, (hereinafter referred to simply as “bellows”) are formed using accordion steel pipes having parallel corrugated walls perpendicular to the axis of the pipes. Bellows can be elongated and contracted, be deflected, or be bent. Thus, bellows have been used in a wide variety of applications, including transportation means such as automobiles, ships, and aircraft, air conditioning, industrial plant equipment, and scientific experiment equipment such as vacuum equipment. In particular, bellows have recently been used as joints in automotive exhaust pipes and function to effectively absorb thermal strain due to high-temperature auto emission and the throb of an engine.
Metallic bellows and flexible tubing are processed by roll forming, hydraulic forming, elastomeric forming, expansion forming, and other various methods, as described in Ukita, “Berouzu oyobi hurekisiburukan no saikin no seizou gijutu (Recent technique of manufacturing bellows and flexible pipes)”, Sosei to kakou, July 1991, 32, 366, pp. 818-824. An optimum processing method is selected according to the size and the application of bellows. FIG. 1 is a schematic view illustrating a hydraulic forming method in one-ridge-forming mode as a method for manufacturing an automotive exhaust bellows pipe. Reference numeral 1 denotes a raw material pipe for forming bellows pipes. After the raw material pipe is placed as illustrated in FIG. 1, the raw material pipe is filled with liquid, which is pressurized to expand the wall of the raw material pipe between a clamp die 3 and a forming die 4. Then, the clamp die 3 is compressed in the axial direction of the pipe to form a ridge 6. These procedures are repeated to manufacture a bellows having many ridges. Although the productivity is low, this method can save die cost as compared with a method of forming a predetermined number of ridges by hydraulic forming in one operation. Furthermore, the method illustrated in FIG. 1 can advantageously utilize a relatively simple die and form any number of ridges having any height, thus producing a bellows having a desired shape and a desired number of ridges.
Raw material pipes to be processed into bellows are generally single or duplex pipes of FCC metal (face centered cubic metal) such as copper or SUS 304 (austenitic stainless steel). The reason that the FCC metal or austenitic stainless steel sheets are used is that other metallic materials are difficult to process into bellows. Although raw material pipes made of the austenitic stainless steel sheets are easily processed into bellows, high-temperature or corrosive gas or liquid flowing through the bellows tends to cause oxidization, high-temperature corrosion, or stress corrosion cracks. In particular, the service temperature of austenitic stainless steel bellows for use in automotive exhaust systems is in the range of 500° C. to 750° C. Thus, the deposition of snow-melting salt sprinkled on roads tends to cause the sensitization of the material, thereby causing high-temperature salt corrosion. Furthermore, although the austenitic stainless steel is excellent in formability, it is expensive because of its high Ni content. Accordingly, there is a demand for an inexpensive material having excellent formability for use in raw material pipes for forming bellows pipes.
In such situations, various types of Ni-free ferritic stainless steel for use in raw material pipes for forming bellows pipes have been devised. For example, Japanese Unexamined Patent Application Publication No. 7-268560 discloses a ferritic stainless steel sheet that exhibits excellent bellows formability achieving the elongation of at least 35% and the r-value of at least 1.5 by specifying the contents of C, Si, Mn, S, Cr, Al, Ti, N, and O. Furthermore, Japanese Unexamined Patent Application Publication No. 8-176750 discloses a ferritic stainless steel sheet that exhibits excellent bellows formability by specifying the contents of C, Cr, Al, N, Si, Mn, Ti, Nb, Mo, Cu, and Ni and the optimum range of the crystal grain size. Furthermore, Japanese Unexamined Patent Application Publication No. 8-188854 discloses a ferritic stainless steel sheet that exhibits excellent bellows formability and excellent high-temperature fatigue properties by specifying the contents of C, Cr, Al, N, Si, Mn, Ti, Nb, Mo, Cu, and Ni. Furthermore, Japanese Unexamined Patent Application Publication No. 9-125208 discloses a ferritic stainless steel sheet that exhibits excellent bellows formability and resists to high-temperature salt corrosion by specifying the contents of C, Cr, N, Ti, and Mo and the optimum range of the surface roughness of a material.
However, the bellows formability of raw material pipes for forming bellows pipes made of the ferritic stainless steel sheets is still insufficient as compared with that of raw material pipes for forming bellows pipes made of austenitic stainless steel sheets. Thus, the ferritic stainless steel sheets are not applicable to bellows having a larger peak height H. The peak height H of a bellows pipe is half the difference between the outside diameter of peak Φ and the outside diameter of bottom A (the outside diameter of a raw material pipe for forming bellows pipes A) of the bellows pipe, as illustrated in FIG. 1. Furthermore, the present fact is that which characteristic of a ferritic stainless steel sheet used as a material of raw material pipes for forming bellows pipes has an effect on the formability of the raw material pipes for forming bellows pipes is poorly understood.
Accordingly, it could be advantageous to provide a ferritic stainless steel sheet exhibiting excellent bellows formability for use in raw material pipes for forming bellows pipes on the basis of the understanding of characteristics required for the ferritic stainless steel sheet as a material for raw material pipes for forming bellows pipes.
We investigated the relationship between the mechanical characteristics of a ferritic stainless steel sheet material and the formability of a raw material pipe for forming bellows pipes for the purpose of developing a raw material pipe for forming bellows pipes having excellent formability. As a result, we found that the formable maximum peak height Hmax (or the maximum outside diameter of peak ΦMAX; hereinafter referred to simply as ΦMAX) correlates with the yield stress YS of a material and with the produce (YS×UE1) of the yield stress and the uniform elongation properties. We also found that the formable minimum peak height Hmin (or the minimum outside diameter of peak ΦMIN; hereinafter referred to simply as “ΦMIN”) correlates with the yield stress YS of a material in bellows forming. We further found that the difference between the formable maximum peak height Hmax and the minimum peak height Hmin, that is, the formable peak height range (or half of (ΦMAX-ΦMIN)) correlates with the yield stress YS of a material and with the product (YS×UE1) of the yield stress and the uniform elongation properties.
The maximum outside diameter of peak ΦMAX and the minimum outside diameter of peak ΦMIN are determined as follows. Ten ridges were continuously formed by hydraulic forming in one-ridge-forming mode, as illustrated in FIG. 1, under the same conditions while the target value of the outside diameter of peak Φ was changed and the outside diameter of bottom A of a bellows pipe was kept constant (50 mm). Each outside diameter of peak of the 10 ridges was measured as φ(1), φ(2), φ(3), . . . , and φ(10). The minimum value of them was designated as φmin. The maximum value of them was designated as φmax. The mean value of them was designated as φav. A bellows pipe satisfying the relation:0.980φav≦φmin≦φav≦φmax≦1.02φavwas considered as an accepted product. Among φavs of accepted products, the minimum φav was defined as the formable minimum outside diameter of peak ΦMIN. The maximum φav of an accepted product that had no crack during the hydraulic forming was defined as the formable maximum outside diameter of peak ΦMAX. The relation implies that variation in the outside diameter of peak Φ is small. In other words, the relation is indicative of forming stability. The relation is particularly effective in evaluating the formable minimum outside diameter of peak ΦMIN, around which the forming has a tendency to be unstable. On the other hand, the formable maximum outside diameter of peak ΦMAX generally depends on the occurrence of a crack in forming and is less affected by variation in the peak height H.
Thus, we provide a ferritic stainless steel sheet for use in raw material pipes for forming bellows pipes. The ferritic stainless steel sheet contains 10% to 25% by mass of Cr and has a yield stress in the range of 300 to 450 MPa and the product of the yield stress and the uniform elongation properties of at least 5200 (MPa·%).
Furthermore, the stainless steel sheet has a thickness of 0.5 mm or less and is to be used in single or duplex raw material pipes for forming bellows pipes for use in automotive exhaust systems. The outside diameter A of the raw material pipes for forming bellows pipes is in the range of 28 to 80 mm. The outside diameter A of the raw material pipes for forming bellows pipes is the outside diameter of steel pipes before the formation of the bellows pipes, as illustrated in FIG. 1. The outside diameter A of the raw material pipes is almost the same as the outside diameter of bottom A of the bellows pipes.
Furthermore, a stainless steel sheet is a ferritic stainless steel sheet for use in raw material pipes for forming bellows pipes having an average crystal grain size of 35 μm or less.
Furthermore, a stainless steel sheet is a ferritic stainless steel sheet for use in raw material pipes for forming bellows pipes having a surface roughness Ra of 0.40 μm or less. The surface roughness Ra is an arithmetical mean roughness measured in accordance with JIS B0601 (1997) in a direction perpendicular to the rolling direction of a steel sheet using a stylus surface roughness tester in conformity to JIS B0651.
Raw material pipes for forming bellows pipes formed of a ferritic stainless steel sheet are inexpensive and exhibit excellent formability. The raw material pipes are therefore suitably used as raw material pipes for forming bellows pipes for use in automotive exhaust systems.